1. Field of the Invention
The present invention relates to an electromagnetic wave resonator that is used for boosting up an electromagnetic field in various frequency ranges such as microwaves, millimeter waves, teraHz waves, infrared radiation, visible rays and ultraviolet radiation, and has dimensions equal to or smaller than wavelengths, and its fabrication process as well as an electromagnetic wave generator device using the same. Among others, the invention relates to an electromagnetic wave resonator formed of a metal material for use in the infrared to ultraviolet light ranges.
2. Description of the Prior Art
An electromagnetic wave resonator is a device essentially required for the generation and amplification of coherent electromagnetic waves, frequency selection, high-sensitivity electromagnetic wave detection due to a boosted-up electromagnetic field, and the development of various nonlinear effects.
In particular, an electromagnetic wave resonator making use of surface waves present at a negative/positive dielectric interface has attracted a great deal of attention as a structure having a particularly large electromagnetic wave generation effect or the like, because a large electromagnetic field may be confined within a small volume.
In present disclosure, the “positive dielectric material” is defined as having a positive value for its real part of dielectric constant, and the “negative dielectric material” as having a negative value for its real part of dielectric constant. The positive dielectric material corresponds to general nonmetallic materials such as glasses, ceramics, semiconductors, polymers, and liquids. According to the above definition, air, other gases and vacuum voids may be called the positive dielectric material too. On the other hand, the negative dielectric material implies that an object comprising a specific material has such properties as described above in a specific frequency zone alone. Typical of this is a metallic material in a frequency range lower than a plasma frequency, say, in a visible ray or infrared range. Besides, there is the mention of materials capable of developing resonance of large lattice vibrations such as silicon carbide and various ionic crystals in the far-infrared to teraHz range, superconducting materials that can be in a superconducting state in the teraHz to microwave range lower in frequency than the superconducting energy gap, and silicon or other semiconductor materials having excited carriers.
The “surface wave” here is understood to refer to an electromagnetic wave mode in which the amplitude of its electromagnetic field has a maximum value at an interface, and which has a distribution profile that attenuates exponentially with an increasing distance from the interface, and propagates along the interface. With a metallic material used as the negative dielectric material, the surface wave in the infrared and visible ray range is called the surface plasmon or surface plasmon-polariton; and with silicon carbide or various ionic crystals used as the negative dielectric material, the surface wave in the teraHz or infrared range is called the surface polariton or surface phonon-polariton.
A typical electromagnetic wave generator using such an electronic magnetic resonator is disclosed in Patent Publication 1 showing an infrared generator device using metal rectangular cavities as a resonator. This device is operable to heat a lattice having an array of metal rectangular cavities thereby inducing surface plasmon resonance in the resonator so that thermal emission of infrared radiation having a specific wavelength is boosted up.
[Problem with Resonator Fabrication]
For a conventional metal rectangular cavity type electromagnetic wave resonator, however, there was the need for achieving resonance at a given wavelength by fabricating a microstructure having a narrow width and a large depth or a high aspect (depth/width) ratio. According to Patent Publication 1, for instance, there is the need for the structure: an aspect ratio of 9.2 with a width of 0.041 μm and a depth of 0.378 μm for the purpose of obtaining a wavelength of 2.52 μm; an aspect ratio of 6.6 with a width of 0.1 μm and a depth of 0.66 μm for the purpose of obtaining a wavelength of 4.0 μm; and an aspect ratio of 7.3 with a width of 0.155 μm and a depth of 1.13 μm for the purpose of obtaining a wavelength of 6.0 μm. In any structure, the width is way too small to achieve with ordinary photolithography, and the aspect ratio is way too high to achieve with ordinary etching or mold transfer. Such a structure would not be easy to achieve because of the need for special contrivances.
[Fundamental Problem with Heat Radiation]
There were some problems arising from the use of heat radiation phenomenon as the principle of generating electromagnetic waves from the resonator. The maximum intensity obtained from heat radiation is strictly limited by Planck's law. Although making the resonator temperature high is essentially necessary for obtaining high-intensity radiation, materials available for that purpose were limited to those having high heat resistance, resulting in many design restrictions. Maintaining such a microstructure having a high aspect ratio as mentioned above in high-temperature environments and repetitive heating/cooling cycles resulted in stringent robustness demands. Generally in electromagnetic wave resonators, there are discrete resonances of various orders appearing unavoidably; however, all of them contributed to radiation by Planck's law, ending up with the inability to radiate a wavelength of a specific order alone. In addition, when an infrared generator device is used with analyzer systems, the luminance of a light source has often been modulated for lock-in detection enabling high-sensitivity detection; however, a conventional light source was slow to respond because of the use of heat as an energy source, rendering efficient analysis difficult.
[Problem Arising from the Fact that there was No Option but to Rely Upon the Heat-Radiation Principle]
So far, the fact that there was no option but to rely upon heat radiation in principle was originated from the rudimental structure of a resonator. Apart from heat radiation, there are other light-emitting mechanisms used for light emitters such as recombination emission of electrons and holes in semiconductors, electroluminescence, cathode luminescence and tunnel emission in inorganic or organic materials, or the like, and for all of them, there is the need of passing currents through the positive dielectric part. In the conventional resonator structure, however, negative dielectric areas surrounding a positive electric area were all linked to and in conduction with one another, making it impossible to apply voltages to the positive dielectric area thereby passing currents through it. For that reason, such a variety of light-emitting mechanisms cannot yet be utilized.
The tunnel light-emission phenomenon is described in Non-Patent Publication 1, but the incorporation of a tunnel light-emission device into a resonator structure is not shown in it. This publication shows light emission experiments at low temperatures; however, many studies made later have revealed that similar light emission takes place even at room temperature.
A physical phenomenon underlying an electromagnetic resonator is resonance that occurs by the reflection of surface waves propagating through a slab waveguide having a positive dielectric film core clad by a negative dielectric material off the end face of the waveguide. While the details of this resonance phenomenon are described in Patent Publication 2 and Non-Patent Publication 2, it has generally such features as mentioned below.
There are multiple modes of surface waves present on the slab waveguide having a positive dielectric core clad by a negative dielectric material, among which the lowest order of surface wave is useful. More specifically, useful is the surface wave of the mode that has an electric field component vertical to the positive/negative dielectric interface and no cutoff frequency. One feature of this surface wave is that it has the same electromagnetic field symmetry as a plane wave propagating in a free space (that refers to a space outside of the waveguide irrespective of whether it is air, a vacuum, a liquid or the like) so that it can easily be excited only by irradiation with the plane wave from the end face of the waveguide or, conversely, an electromagnetic wave can be emitted from the end face into the free space. Another feature is that its wavelength λP is shorter than vacuum wavelength λO when it propagates in a vacuum, and the smaller the core thickness T, the shorter the wavelength λP becomes. Specific relations between the vacuum wavelength λO and the wavelength λP of the surface wave are shown in formula form in Patent Publication 2.
Once such a surface wave has arrived at the end face of the waveguide, it is reflected off there, again going back to the waveguide. The then reflectivity grows higher as the core thickness T gets smaller. What phase relation the incident wave is reflected in is determined depending on the state of the end face. When the front of the end face is covered with the dielectric material (called the open end), the incident wave is reflected in such a phase relation that the electric field reaches a maximum at the interface. Accordingly, when both ends in the propagation direction remain open, there is going to be resonance of surface waves having discrete wavelengths λP such that the waveguide length L in the XZ section matches (1/2)λP, (2/2)λP, (3/2)λP, . . . . It follows that such a waveguide works as a resonator for surface waves having those wavelengths. Strictly speaking, however, the electric field does not precisely reach a maximum at the open end: there are more or less deviations. For this reason, design should be carried out with some correction (the same as end correction famous for columnar resonance). If Maxwell equations are strictly calculated by proper numerical calculations to find out a condition under which the electromagnetic energy built up within and near the positive dielectric core reaches a maximum, it is then possible to precisely determine a waveguide length just where the electromagnetic waves of the desired vacuum wavelength λO resonate.
What has been described just above is the resonance phenomenon disclosed in Patent Publication 2, but the practical assumption of Patent Publication 2 was that the slab waveguide takes on a linear form in the XZ section. In other words, this publication does not give any suggestion about whether or not resonance is maintained when the slab waveguide has a shape other than the linear one.